Method for depositing copper or a copper alloy

ABSTRACT

The present invention is related to the fabrication of at least a part of a Cu-containing layers or a Cu-containing pattern used for the electrical connection of active or passive devices as well as integrated circuits. Such Cu-containing patterns and/or layers are formed on an activated surface of a substrate by means of immersion of said substrate in an electroless Cu plating solution. Such a solution typically comprises: a source of copper Cu (II) ions; a reducing agent; an additive to adjust the pH of said aqueous solution to a predetermined value; and a chemical compound for complexing said Cu ions, said chemical compound having at least one part with chemical structure COOR1-COHR2, R1 being a first organic group covalently bound to the carboxylate group (COO), R2 being either hydrogen or a second organic group. Further disclosed is a method for depositing Cu on an activated surface and particularly on an activated surface of a Cu diffusion barrier layer.

This application claims priority to U.S. Provisional Application No.60/116,110 filed on Jan. 15, 1999, and European Application No.99870077.7 filed Apr. 29, 1999.

FIELD OF THE INVENTION

The present invention is related to metal deposition processes as usedfor instance for the formation of conductive patterns connecting activeor passive devices as well as integrated circuits. Particularly, suchconductive patterns can be formed at least partly by means of anelectroless deposition technique.

BACKGROUND OF THE INVENTION

Currently, copper is being introduced in ULSI metallization schemes as areplacement for aluminum due to its lower resistivity and betterelectromigration resistance. Copper electroplating is the most populardeposition technique today. To avoid contamination of the surroundinginsulating layers and/or the substrate, copper is mostly deposited on aCu diffusion barrier layer. However, to allow electroplating, first aconductive seed layer has to be formed on top of the barrier layer(s) inorder to get reliable electroplated copper deposition. Usually, asputtered copper layer is used for this purpose. However, for dualdamascene type processing with very high aspect ratio openings indielectric layers such as trenches, via holes and contact holes, thestep coverage of sputtered (classical or by means of an Ion MetalPlasma) barrier layers and Cu seed layers is expected to become thelimiting factor for subsequent filling with e.g. electroplated copper.As a consequence, alternative deposition routes for copper seed layerformation can be attractive for future device technologies. Electrolesscopper deposition has the potential of becoming a viable alternativebecause it can deliver high step coverage depositions at a very lowcost. The principle of electroless metal deposition is based on thegeneration of electrons at a catalytically active or an activatedsurface in contact with a solution of metal ions in the presence of asuitable sacrificial electron donor. These electrons are capable ofreducing the metal ions leading to the deposition of the metal on theactivated surface. Because this process does not occur on non-activatedlayers, the resulting deposition technique is inherently selective.Furthermore, in principle, it should be easy to integrate such processin currently available copper electroplating tools (which are already onthe market or will be so in the near future), provided amongst othersthat the electroless plating solution is stable at room temperature forat least about two weeks and the process margins of the plating solutionare not too tight. Nowadays most electroless copper plating solutions donot meet these specifications. They have often a limited stability andcan only be effectively used in a limited pH range which makes them verysensitive for slight variations in the composition of the platingsolution as such variations result in small variations in the pH butoften lead to a large decrease in the deposition rate.

Moreover, most available electroless copper plating baths do not fulfillthe stringent requirements for copper plating in sub-micron high aspectratio features onto the typical Cu diffusion barrier layers used in ULSIprocessing. Typical barrier layers are Ti, TiN, Ta, WN_(x), TaN, Co andany combination thereof, and other Cu diffusion barrier layers known inthe art. One of the problems of electroless copper deposition on barrierlayers and particularly on e.g. TiN is the evolution of copious amountsof hydrogen gas which is detrimental for the quality of the copper layerformed because it leads to severe blistering of the copper layer. Theuse of cyanide as a hydrogen suppressor, as practiced in many commercialcopper plating solutions, is not acceptable in this business due tosafety issues. Another problem related to the state-of-the-art platingsolutions is the bad adhesion of electroless deposited copper on suchbarrier layers.

Moreover, most electroless copper plating solution compositions arebased on salts containing mainly sodium as the counterion. These highlevels of sodium ions in the plating solutions can introduce severereliability problems, particularly when sodium reaches the semiconductordevice junctions, as this is known to be a production yield killer insemiconductor device manufacturing. Therefore as a further requirementthe level of sodium ions in the plating solution should be very limitedor negligible.

Nowadays, electroless copper plating solutions often use EDTA as thecomplexing agent and formaldehyde as the reducing agent. The complexingagent is required to keep Cu(II) ions in solution at the relatively highpH values at which formaldehyde operates as a reducing agent. Recently,the trend is to move away from strong complexants such as EDTA. Due totheir strong complexing power for many metal ions, more stringentrequirements for the environment are expected for plating solutionsbased on strong complexing agents. Consequently, there is a need forother more environmentally acceptable complexing agents.

AIMS OF THE INVENTION

It is an aim of the invention to provide a plating solution forelectroless copper deposition which is substantially sodium free (lowlevel sodium) and which comprises an environmentally friendly complexingagent. The plating solution should have a long life-time after make-up(thermal stability) of at least two weeks at room temperature and shouldbe easy to replenish to keep the plating characteristics well withinspecifications over the plating periods. Furthermore, slight variationsin the composition of the plating solution may cause only smallvariations in the Cu deposition rate on an activated surface of asubstrate immersed in the plating solution.

It is another aim of the invention to provide a method to formCu-containing layers on an activated surface of a substrate by means ofan electroless deposition technique using the plating solution of thepresent invention. This plating solution and deposition method should besuch that the formation of hydrogen gas during the plating period isavoided or severely limited. Particularly, this method has to result ina sufficiently high deposition rate at low temperatures.

A further aim of the invention is to provide a Cu seed layer on adiffusion barrier layer, said diffusion barrier layer may actsimultaneously as a wetting layer. Particularly when a Cu-containinglayer is deposited on a barrier layer, the Cu-containing layer has to beformed with good adhesion on the diffusion barrier layer. Seed layershave typically a thickness below 300 nm.

Still a further aim of the invention is to provide a Cu plating solutionsuitable to form a relatively thick Cu-containing layer on a barrierlayer or on a seed layer. This is particularly useful to fill upopenings with high aspect ratios in insulating layers such as via holes,trenches and contact holes like e.g. in damascene architectures. TheCu-containing layers formed have typically a thickness between 200 nmand 2 μm.

SUMMARY OF THE INVENTION

The present invention is related to the fabrication of at least a partof a Cu-containing layers or a Cu-containing pattern used for theelectrical connection of active or passive devices as well as integratedcircuits. Such Cu-containing patterns and/or layers can be formed on anactivated surface of a substrate by means of immersion of said substratein an electroless plating solution. Therefore, in an aspect of thepresent invention, an aqueous solution for electroless deposition of Cuon a substrate is disclosed, said solution comprising:

a source of copper Cu (II) ions;

a reducing agent;

an additive to adjust the pH of said aqueous solution to a predeterminedvalue; and

a chemical compound for complexing said Cu ions, said chemical compoundhaving at least one part with chemical structure COOR1-COHR2 (as can beseen in FIG. 2a)), R1 being a first organic group covalently bound tothe carboxylate group (COO), R2 being either hydrogen or a secondorganic group. Examples of such first or second organic groups arehydrocarbon groups, while for instance the chemical compound forcomplexing the Cu ions can be selected from the group consisting ofdiethyltartrate, diisopropyltartrate and diethyllactate. The pH of theplating solution ranges typically from 11 or 11.5 to 13.5, while thetemperature at which the solution can be applied ranges from 10 to 50degrees C. or, 45 degrees C. or below, or from room temperature to 40degrees C. Examples of a reducing agent are formaldehyde,paraformaldehyde, hydrazine, amine boranes, alkali metal borohydrides,alkali metal hypophosphites or a derivative of one of the aforementionedreducing agents.

In an embodiment of the invention, a Cu plating solution is disclosedbased on an organic based complexing agent, wherein the ratio betweenthe concentration of said source of copper Cu (II) ions and theconcentration of said complexing agent in said solution is in the rangefrom 1/5 to 5/1 or from 1/10 to 10/1 or from 1/25 to 25/1.

In another embodiment of the invention, a Cu plating solution isdisclosed wherein the complexing agent is a chemical compound withchemical structure COOR1-CHOH—CHOH—COOR1, R1 being an organic groupcovalently bound to the carboxylate group (COO). For instance,hydrocarbon groups can be used as organic groups. Particularly, achemical compound selected from the group consisting of diethyltartrate,diisopropyltartrate and dimethyltartrate can be selected.

In another aspect of the invention, a method is disclosed for forming aCu-containing layer on a substrate comprising the steps of:

preparing an aqueous solution comprising a source of copper Cu (II)ions, a reducing agent, a chemical compound for completing said Cu (II)ions, said chemical compound having at least one part with chemicalstructure COOR1-COHR2, R1 being an organic group covalently bound to thecarboxylate group (COO), R2 being either hydrogen or an organic group,and an additive to adjust the pH of the solution to a predeterminedvalue;

immersing said substrate in said aqueous solution for a predeterminedperiod to thereby form said Cu-containing layer at least on an activatedsurface of said substrate. For instance, this Cu-containing layer can beformed on a Cu diffusion barrier layer formed on the substrate.

In an embodiment, a method for filling an opening in an insulating layeris disclosed, wherein, after forming at least one opening in aninsulating layer formed on a substrate, a barrier layer can be formed onat least one inner wall of said opening. Examples of such openings arevia holes, contact holes, and trenches. Examples of such barrier layersare layers of Ti, or TiN, or Ta, or tungsten nitride, or TaN, or Co or acombination thereof. Particularly this barrier layer can also act as awetting layer like e.g. a TiN layer with a Ti layer thereon. ACu-containing metal, i.e. an alloy of or pure Cu, is deposited using theelectroless plating solution of the present invention. This electrolessdeposition can be performed in a chamber of a electrodeposition tool.The electroless deposition can be performed in at least one depositionstep to thereby completely fill the openings. Alternatively, first athin Cu-containing seed layer can be formed using the electrolessdeposition method of the present invention, thereafter a secondCu-containing metal can be deposited on said seed layer using adifferent Cu deposition technique as e.g. electroplating of Cu tothereby completely fill the openings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the deposition rate of a tartrate²⁻ ions based salt Cuplating solution versus the pH value of the solution.

FIG. 2 depicts three different chemical structures. According to thepresent invention, in FIG. 2a) a chemical compound for complexing Cuions is depicted having at least a part with chemical structureCOOR1-COHR2, R1 being a first organic group covalently bound to thecarboxylate group (COO), R2 being either hydrogen or a second organicgroup, while in FIG. 2b) a chemical compound with chemical structureCOOR1-CHOH—CHOH—COOR1 is depicted, and FIG. 2c) depicts the chemicalstructure of diethyltartrate.

FIG. 3 depicts, according to an embodiment of the invention, thedeposition rate of a diethyltartrate based Cu plating solution versusthe pH value of the solution.

FIG. 4 depicts, according to an embodiment of the invention, thethickness of the deposited Cu layer versus deposition time usingelectroless deposition of three different plating solutions being atartrate²⁻ ions based salts plating solution comprising CuSO₄ in anamount of 0.0144 mol/l and a tartrate²⁻ ions based salt in an amount of0.0166 mol/l (42); a tartrate²⁻ ions based salts plating solutioncomprising CuSO₄ in an amount of 0.0443 mol/l and a tartrate²⁻ ionsbased salt in an amount of 0.0499 mol/l (43); a diethyltartrate basedplating solution comprising CuSO₄ in an amount of 0.0288 mol/l anddiethyltartrate in an amount of 0.1461 mol/l (41).

FIG. 5 depicts, according to an embodiment of the invention, thethickness of the deposited Cu layer versus the deposition temperatureusing electroless deposition of a diethyltartrate based electrolessplating solution.

FIG. 6 depicts, according to an embodiment of the invention, thedeposition rate of a diethyltartrate based plating solution versus theconcentration of CuSO₄ in the solution for two different compositions ofthe solution being a solution with diethyltartrate in an amount of0.1461 mol/l (61); and a solution with diethyltartrate in an amount of0.256 mol/l (62);.

FIG. 7 depicts, according to an embodiment of the invention, Cu seedlayers formed using a electroless deposition of a diethyltartrate basedplating solution, on top of a Ti/TiN stack. This Ti/TiN stack is formedon wafers (substrates) with single damascene trench test structureshaving widths down to 0.4 micron and aspect ratio of 2.5.

FIG. 8 depicts the same single damascene trench test structures as inFIG. 5 wherein these trench structures are completely filled with Cuusing an electroplating technique.

DETAILED DESCRIPTION OF THE INVENTION

In relation to the appended drawings the present invention is describedin detail in the following. It is apparent however that a person skilledin the art can imagine several other equivalent embodiments or otherways of executing the present invention, the spirit and scope of thepresent invention being limited only by the terms of the appendedclaims.

The use of tartrate²⁻ ions based salts as complexing agents forcopper(II) ions in electroless plating solutions is known for manyyears. However, only the sodium, potassium and NaK (Rochelle salt) salthave been used extensively in electroless copper plating solutioncompositions. A disadvantages of the latter type of plating solution isthe very low deposition rate at room temperature. Another disadvantageis the sensitivity of these state-of-the-art complexing agents for smallvariations in the composition of the plating solution which directlyaffects the pH of the solution. Therefore even if one succeeds inobtaining a more or less stable initial solution, while platingproceeds, a small change in the composition and consequently in the pHof the solution caused by the deposition of Cu causes dramatic changesin the deposition rate due to the limited width of the pH window atwhich the deposition rate is more or less constant (as e.g. in FIG. 1).

A further disadvantage is that electroless deposition using a platingsolution based on tartrate²⁻ ions based salts leads to hydrogen gasevolution which not only prevents deposition of layers with a thicknessabove 150 nm, but also thinner layers (below 150 nm) are of bad qualitydue to hydrogen inclusion in the deposited layer.

A further disadvantage of electroless deposition using a platingsolution of these tartrate²⁻ ions based salts containing mainly sodiumor potassium as counterion is that these counterions are highly mobileand easily migrate to the junction level of the semiconductor deviceswhich is detrimental for the reliability of these devices.

Moreover, it should be stated that successful plating of electrolesscopper on an arbitrary surface of a substrate involves both cleaning ofthe surface and activation with Pd nuclei prior to copper plating.

In a preferred embodiment of the invention, a method is disclosed forforming a Cu-containing layer on an activated surface of a Cu-diffusionbarrier layer formed on a substrate comprising the steps of:

preparing an aqueous solution comprising a source of copper Cu (II)ions, a reducing agent, a chemical compound for complexing Cu (II) ions,said chemical compound having chemical structure COOR1-CHOH—CHOH—COOR1(as in FIG. 2b)), R1 being an organic group covalently bound to thecarboxylate group (COO), and an additive to adjust the pH of thesolution to a predetermined value;

immersing said substrate with said Cu-diffusion barrier layer in saidaqueous solution for a predetermined period to thereby form saidCu-containing layer at least on said activated surface of saidCu-diffusion barrier layer.

At least for the purpose of this disclosure, an organic tartrate isdefined as a chemical compound with chemical structureCOOR1-CHOH—CHOH—COOR1, R1 being an organic group covalently bound to thecarboxylate group (COO). For instance, these organic groups can behydrocarbon groups. Examples of such organic tartrates arediethyltartrate (FIG. 2c)), diisopropyltartrate and dimethyltartrate.

In order to avoid copper(II)hydroxide deposition at high pH values, anorganic tartrate is added for complexing the Cu(II) ions. Particularly,diethyltartrate, is used. The organic tartrates are characterized by adifferent complexation behaviour with Cu(II) ions as compared to ionictartrate, i.e. tartrate²⁻ ions based salts. Although the precise natureof the complexation behaviour is not fully understood, experimentsdemonstrate that deposition using the plating solution of the presentinvention, i.e. an organic tartrate based solution, unexpectedlyovercomes or at least substantially limits the hydrogen evolution duringthe deposition process, probably due to the different complexingbehavior, particularly when a deposition is performed on a barrierlayer. It is believed that when adding an organic tartrate to thesolution, in contrast to adding tartrate²⁻ ions based salts, nocarboxylate ions are created. As a consequence, the complexation ofCu(II) ions occurs mainly at higher pH values. Probably thiscomplexation is mainly, but not limited hereto, based on at least onehydroxyl group of the organic tartrate, particularly on thecorresponding anion.

CuSO₄.5H₂O can be used as a source of copper Cu (II) ions or othersources known in the art. Formaldehyde acts as a sacrificial electrondonor, i.e. a reducing agent. The invention is in no way limited to theuse of formaldehyde as sacrificial electron donor. Formaldehyde vaporsare a potential health liability because of suspected carcinogenity.However, one could opt to use the solid form, i.e. paraformaldehydewhich is less dangerous. One could also opt to add paraformaldehyde tothe solution provided that first the plating solution is brought to ahigher pH. This probably further limits health risks.

The operation range for the pH of this plating solution is typicallybetween pH 11.5 and 13.5. The correct pH value is adjusted by additionof an additive like e.g. tetra-N-methylammoniumhydroxide (Me₄NOH). Otherexamples are the alkali metal hydroxides or others known in the art.

The substrate can be at least a part of a partly processed or a pristinewafer or slice of a semi-conductive material, like e.g. Si or GaAs or Geor SiGe, or an insulating material, like e.g. a glass slice, or aconductive material. Said substrate can comprise a patterned insulatinglayer. Particularly, in case said substrate is a partly processed waferor slice; at least a part of the active and/or passive devices canalready be formed and/or at least a part of the structuresinterconnecting these devices can be formed.

Examples of Cu diffusion barrier layers are Ti, TiN, Ta, tungstennitride, TaN, Co or any combination thereof. A more particular exampleof such a barrier layer is TiN. When stating that the hydrogen evolutionis substantially limited during deposition on a barrier layer, thismeans that on such barrier layers high quality Cu-containing layers witha thickness of at least 150 nm or at least 300 nm can be formed. Alsothicker layers can be formed, e.g., layers with a thickness ranging upfrom 1 μm or even up to 2 μm.

The electroless plating of copper on barrier layers involves bothcleaning of the barrier layer surface and activation of the cleanbarrier layer with Pd nuclei prior to copper plating. For instance,cleaning of a TiN surface can be accomplished with dilute HF solutionsin order to remove surface nitrided titanium dioxide species. Othercleaning procedures have been described in the literature. Activation isaccomplished by treatment of the clean TiN surface with a Palladiumactivator solution, typically containing PdCl₂ and HCl in aqueoussolution. Optional additives are HF and/or acetic acid, as known in theart. It is understood that each process step should be followed by anadequate rinse, for instance with DI water, as usually required in theart. In some instances, depending on the quality of the TiN surface,additional drying after either pre-clean or activation step, or afterboth steps, can improve the electroless copper layer quality.

Electroless plating can be performed at temperatures up to 55° C.However, the stability of the plating solution deterioratessubstantially when the plating solution is kept at temperatures aboveabout 45° C. For instance, the stability of the solution was found to bereduced to approximately 1 day at 40° C. as compared to more than 30days at room temperature. Consequently, preferably the temperature rangefor electroless plating with the plating solution of the presentinvention is between 20 and 40° C. The copper deposition rate follows aperfect Arrhenius behavior in function of the temperature of thesolution with an activation energy of 56.13 kJ mol⁻¹ (41) (see FIGS. 4and 5)) in the temperature range between 20 and 55° C. As depicted inFIG. 5 deposition rates are 31.6 nm min⁻¹ at 34° C. and 46.4 nm min⁻¹ at40° C., respectively. In case a conventional tartrate²⁻ ions based saltplating solution is used then the deposition rates (42) (43) aresubstantially lower.

As a best mode embodiment of the invention, a plating solution isdisclosed having the following composition: Cu²⁺ (as CuSO₄.5H₂O) (0.029M), diethyltartrate (0.146 M), and formaldehyde (0.67 M). The pH isadjusted with [Me₄N]OH to an optimum value of 12.5. The procedure formaking up the plating bath involves mixing of the Cu(II) anddiethyltartrate stock solutions, adding water to almost final volume,first pH adjustment to 12.5; addition of formaldehyde, reestablishingthe pH value to 12.5, and finally adding water to the final solutionvolume. The stability of this solution at room temperature exceeds 30days which is an important improvement compared with conventionalplating solutions which have a limited stability of typically one weekor at most two weeks. Decrease of the copper ion concentration in theplating solution results in lower deposition rates as can be seen inFIG. 6. However, the copper layer characteristics do not changeappreciably by changing the copper concentration and/or by changing theorganic tartrate concentration (61) (62) in the plating solution of thepresent invention. Also, the formaldehyde concentration is not thatcritical for obtaining a good copper quality; however too large anexcess should be avoided because hydrogen evolution is favored at higherformaldehyde concentration levels, i.e. more than about 1 M. Thisdecreased sensitivity of the precise amounts in the plating solutioncomposition and the increased stability of the organic tartrate basedplating solution makes this plating solution suitable for industrialapplication.

Further according to this example, a Cu-containing layer is formed on aTi/TiN stack. The Ti/TiN was deposited by means of a physical vapordeposition (PVD) process. Deposition can also be executed by means ofALCVD. The following characteristics for the plating process and thedeposited copper layers are obtained. In the pH range between 12.0 and13.0, as in FIG. 3, the deposition rate amounts to approximately 13 nmper minute at room temperature (21° C.) and does not change withdeposition time (thickness increases linearly with deposition time).Specific resistivity values between 4.10 and 4.65 μΩcm are obtained fora layer thickness in the range of 275 to 300 nm. Thinner layers ofelectroless copper have higher resistivity values, as is also known fromcopper films deposited by PVD and especially by chemical vapordeposition (CVD) methods. For instance, a 110 nm thick electrolesscopper film shows a specific resistivity of 5.63 μΩcm. However,deposition at higher temperatures results in a significant decrease ofthe specific resistivity of electroless copper layers deposited with theplating solution of the present invention. For instance, a copper filmof 160 nm deposited at 40° C. shows a specific resistivity of only 4.0μΩcm. Typical sheet resistance uniformity over a 6 inch wafer with a PVDdeposited Ti/TiN stack is around 6.6% (standard deviation, 1 sigma, asmeasured over 49 points by a four-point probe measurement).

The deposition of Cu films as seed layers for Cu electroplating (withthe ECD technique) is compared for an electroless plating methodaccording to the best mode embodiment of the invention, and for copperfilms deposited with the PVD technique. Electroless Cu seed layers andECD Cu films were deposited in an EQUINOX tool as commercially availablefrom the company SEMITOOL. 6″ silicon wafers with different widthtrenches are used. The trenches are etched in an oxide slayer formed onthe silicon wafer. Trench depth was 1 μm and 1.2 μm, for different lotsof wafers. 1 μm thick ECD Cu films were deposited on 80 nm Cu seedlayers (electroless or PVD) on 15 nm/60 nm of Ti/TiN layers. ElectrolessCu seed layers are deposited in Equinox at standard conditions fromNa-free electroless bath followed by a RTP anneal (350 C., 2 min, N2). Asecond anneal step was performed after ECD copper deposition. Thereafteran edge bead removal step, CMP, and post-CMP clean (H₂O) were done.Wafers were electrically tested after CMP. Some of the wafers were usedfor FIB SEM (FEI200) and SEM (Philips XL30) analysis. The copper seedlayers deposited from the Na-free electroless copper bath, according tothe best mode embodiment of the invention, had good uniformity andconformality in trenches with aspect ratio of at least up to 3. ECDcopper deposited on electroless Cu seed layers showed excellent filingcapability in 1.2 μm as well as in 1 μm deep trenches. Trenches withdifferent widths (from 0.3 μm up to 10 μm) were electrically measured.The results show that electroless Cu seed layers can be deposited intrenches with an aspect ratio of at least up to 3.5 (1.2 μm: 0.35 μm).ECD Cu films plated on electroless Cu seed layers have reasonableeffective specific resistivity about 2.3 μΩ-cm for trenches with widthfrom 0.35 μm to 0.7 μm and 2.12 μΩ-cm for 3 μm width trenches. Similarresults are obtained for ECD Cu films deposited on PVD Cu seed layers.

Further according to the method of the present invention, a method forfilling an opening in an insulating layer is disclosed, wherein, afterforming at least one opening in an insulating layer formed on asubstrate, a barrier layer is formed on at least one inner wall of saidopening. A Cu-containing metal, i.e. an alloy of or pure Cu, isdeposited using the electroless plating solution of the presentinvention. This deposition can be performed in a chamber of aelectroplating tool. The obtained Cu layer can be used as seed layer (asin FIG. 7) for the deposition of electroplated copper on both blanketwafers, i.e. without openings, and wafers with single damascene trenchtest structures with widths down to 0.4 micron and with an aspect ratioof 2.5. For instance on this seed layer, a second Cu-containing metallayer can be formed by means of electroplating. As an example, excellentfilling (FIG. 8) is obtained with such a combination of layers on top ofa Ti/TiN stack. Adhesion is adequate for a layer thickness as typicallyrequired for seed layer applications. Anneal in inert atmosphere(nitrogen) of the seed layer prior to copper electroplating results ingood adhesion of the total layer with a thickness up to at least 1.1micron, but the invention is in no way limited to this thickness range.

The method of the present invention can be used for the fabrication ofintegrated circuits, particularly in sub 0.35 μm CMOS or BiCMOSprocesses. Particularly these integrated circuits comprise interconnectstructures wherein during the process of forming said interconnectstructures openings, e.g. via openings or contact openings or trenches,with a sub 0.5 μm diameter and with high aspect ratios, i.e. typicallywith an aspect ratio of 2:1 or higher, have to be filled. The method ofthe present invention allows at least forming of high qualityCu-containing seed layers in a reliable way but also complete filling ofthese openings can be obtained. Particularly in an embodiment of theinvention an integrated circuit comprising an interconnect structure isdisclosed, wherein the process of forming said interconnect structurecomprises the method of the present invention for creating Cu-containingmetal layers on a Cu diffusion barrier layer.

What is claimed is:
 1. An aqueous solution for electroless deposition ofCu on a substrate comprising: a source of copper Cu (II) ions; areducing agent; an additive to adjust the pH of said aqueous solution toa value between 11 and 13.5; and a chemical compound for complexing saidCu ions, said chemical compound having at least one part with chemicalstructure CHOH—COOR1, R1 being selected from the group consisting ofdimethyl, diethyl, and diisopropyl covalently bound to the carboxylategroup (COO).
 2. The solution as recited in claim 1, wherein thetemperature of said aqueous solution for electroless deposition of Cu isabout 45 degrees C. or below.
 3. The solution as recited in claim 1,wherein said reducing agent is selected from the group consisting offormaldehyde and paraformaldehyde, said reducing agent having aconcentration of less than or equal to about 1.0M.
 4. The solution asrecited in claim 1, wherein said chemical compound is selected from thegroup consisting of diethyltartrate, diisopropyltartrate anddiethylacetate.
 5. An aqueous solution for electroless deposition of Cuon a substrate comprising: a source of copper Cu (II) ions; a reducingagent; an additive to adjust the pH of said aqueous solution to a valuebetween 11 and 13.5; and a chemical compound for complexing Cu (II)ions, said chemical compound having chemical structureCOOR1-CHOH—CHOH—COOR1, R1 being selected from the group consisting ofdimethyl, diethyl, and diisopropyl covalently bound to the carboxylategroup (COO).
 6. The solution as recited in claim 5, wherein saidreducing agent is selected from the group consisting of formaldehyde andparaformaldehyde.
 7. The solution as recited in claim 5, wherein saidchemical compound is selected from the group consisting ofdiethyltartrate, diisopropyltartrate and dimethyltartrate.
 8. A methodfor forming a Cu-containing layer on a substrate comprising the stepsof: preparing an aqueous solution comprising a source of copper Cu (II)ions, a reducing agent, a chemical compound for complexing said Cu (II)ions, said chemical compound having at least one part with chemicalstructure CHOH—COOR1, R1 being selected from the group consisting ofdimethyl, diethyl, and diisopropyl covalently bound to the carboxylategroup (COO), and an additive to adjust the pH of the solution to a valuebetween 11 and 13.5; and immersing said substrate in said aqueoussolution for a period of time of between 2 and 110 minutes to therebyform said Cu-containing layer at least on an activated surface of saidsubstrate.
 9. The method as recited in claim 8, wherein saidCu-containing layer is formed on an activated surface of a Cu diffusionbarrier layer formed on the substrate.
 10. The method as recited inclaim 9, wherein said Cu diffusion barrier layer is at least one layerselected from the group consisting of a Ti layer, a TiN layer, a Talayer, a tungsten nitride layer, a TaN layer, and a Co layer.
 11. Themethod as recited in claim 8, wherein said chemical compound is selectedfrom the group consisting of diethyltartrate, diisopropyltartrate anddimethyltartrate.
 12. The method as recited in claim 8, wherein afterpreparing said aqueous solution, said substrate and said solution areintroduced in a chamber of an electroplating tool.
 13. The method asrecited in claim 12, wherein after immersing said substrate in saidaqueous solution, a Cu-containing metal is deposited on saidCu-containing layer formed on said substrate in a chamber of saidelectroplating tool by means of electroplating.
 14. The method asrecited in claim 13, wherein the thickness of said Cu-containing layeris between about 40 nm and about 2000 nm.
 15. The solution as recited inclaim 1, wherein the pH of said aqueous solution is in the range of fromabout 11.5 to about 13.5.
 16. The solution as recited in claim 3,wherein said reducing agent has a concentration of below about 1 M. 17.The method as recited in claim 8, wherein the pH of said aqueoussolution is the range of from about 11.5 to about 13.5.
 18. The methodas recited in claim 14, wherein the thickness of said Cu-containinglayer is between about 40 nm and about 600 nm.
 19. The method as recitedin claim 18, wherein the thickness of said Cu-containing layer isbetween about 40 nm and about 250 nm.